Posts Tagged ‘petrography’

The deadline for Accretionary Wedge #43 – “my favorite geological illustration” was extended, which finally kicked me into deciding what to post. There are a number of diagrams I love that people have already used (Erik’s bubble figure, the Wooster fun with chemographic diagrams, MK’s subduction zone — which I’ve drawn on tables at Italian restaurants & on onsies at baby showers). And then there were the ones that were finalists (Tharp ocean map is awesome, the USGS Volcanic Hazards poster is basically reproduced by students during intro classes, the different types of silicates), but in the end, I had to go for what I truly know — metamorphic thin sections. Though photomicrographs are gorgeous, to truly “see” what’s going on with textures, you need to draw them by hand. The old-fashioned pen & ink drawings draw your eyes to the key features — ah, for the days when every department had an in-department scientific illustrator.

The following illustrations are of the progressive syntectonic metamorphism of a volcanic graywacke from New Zealand. The original illustrations are from Best (1982): Igneous and Metamorphic Petrology (W. H. Freeman, San Francisco).

original volcanic rock

burial of our volcanic rocks, which turns up the heat & pressure a bit:

breakdown of some of the hydrous material, recrystallization of material to start to form definite foliation

rock continues to be buried, which increases the amount of metamorphism:

continued breakdown of hydrous phases; beginning of segregation into "felsic" vs. "mafic" lithologies

as metamorphism continues, we finally get to the “good stuff” i.e. garnet 🙂

down to just muscovite & biotite as hydrous phases, with a higher mode of anhydrous garnet + garnet + oligoclase dominating; segregation is more pronounced

My students don’t really appreciate my insistence that they have to draw fields of view during mineralogy & petrology, but the process really helps them “see” what’s going on so much better. And though most of them are not in the running to become scientific illustrators in the long run, I do really enjoy grading those labs.

I’m going to aim for getting this done while its still Tuesday locally, but things around here have been a bit crazy due to the rather high snow amounts that fell Sunday / Monday.

This week is also a bit different in that we’re going to be looking at two minerals instead of one and both minerals are opaques. Opaque minerals absorb all of the light that is coming up from the light source below and appear black both in PPL and XPL. Sometimes, we can tell what they are (or at least have a good guess) based on crystal habit. But usually, its not obvious and we need to use either reflected light microscopy or some more serious analytical technique (SEM, EMP, etc.).

I haven’t gotten to reflected light microscopy yet in my series on how to use the microscope (how light behaves & PPL are up), but its coming eventually. If you’re not up on your reflected light techniques, don’t worry – you’re not alone. Reflected light is frequently not even taught in normal mineralogy courses (sorry to out you Kim, but I don’t remember it when I took min & no professor I’ve worked with as a TA taught it). So unless your prof had a love of economic mineralogy, you may not have spent time looking at opaques to discern what they are.

Magnetite in reflected light: pale yellow to light-brown to grey in color (color variation is very minute in reflected light).

Hematite in reflected light: white to white-blue to blue in color. Difficult to differentiate from magnetite. Frequently present as an alteration product, so present as rims or within fractures that may be narrow & difficult to ID.

The ilmenite host (pink-brown) has fine- and coarse-grained haematite (white, bottom left) exsolution bodies oriented along the (0001) direction of the ilmenite. The coarse-grained ilmenohaematite has fine ilmenite exsolution within it. Rutile (lilac-grey, left) is oriented perpendicularly to the crystallographic orientation of the main exsolution bodies. http://www.smenet.org/opaque-ore/08f.jpg

Tabular haematite (blue) and subhedral pyrite (pale yellow-white, centre bottom) are intergrown with chalcopyrite (yellow). The relief difference between the harder haematite and softer chalcopyrite can be seen clearly (centre right). http://www.smenet.org/opaque-ore/21b.jpg

Two detrital grains of ilmenohaematite (centre, right centre) comprise exsolution bodies of ilmenite (brown) in a haematite host (white). The right-hand ilmenohaematite has an ilmenite-free haematite overgrowth (left margin). Very poorly crystalline haematite (green-white, top centre) is poorly polished and is a pseudomorph after an original ferromagnesium mineral, whereas a single crystal of haematite (top left) is well polished.http://www.smenet.org/opaque-ore/42f.jpg

Last week, we had a basic intro to how light behaves. This week, we’re going to step it up and talk about what we can observe in plane light or PPL.

What kind of things are we going to talk about this week?

Becke Line

relief

cleavage

color / pleochroism

There are few ways to look at minerals, such as immersed in an oil of a specific refractive index. (A few of the images I’m going to use are going to refer to oil immersion.) However, today the most common way to examine minerals today is in thin section. I’ll give a few arguments for why in a few posts. But let’s quickly talk about what a thin section actually is & how you make one. First, you need to collect a rock & cut it into a domino with a diamond embedded rock saw. Second, you “glue” the domino to a glass slide with epoxy. Third, you cut off the majority of the domino and then grind down until you only have 30 microns of rock left on the glass slide. At this point, you have a choice: attach a cover slip or continue to polish the slide down till its smooth enough for SEM or EMP analysis. The example below & these instructions (with pictures for each stage!) are for a covered thin section (from Dave Hirsch).

The epoxy has a specific refractive index (1.54 – 1.56 usually), which is important when talking about the Becke Line.

The Becke Line depends on how light refracts when it intersects a mineral. Remember that the item that has the refractive index will cause light to bend inwards towards the normal. With minerals, we use a substance that has a known refractive index (either an oil or the epoxy at the edge of the thin section) and then test for which way the light refracts with it. Focus the microscope while looking at the mineral boundary. You may find this works better if you lower the amount of light going through the microscope. While looking at the mineral boundary, lower the stage of the microscope. This will increase the focal length and cause the brighter of the two boundary lines to move either inwards or outwards. Inwards = mineral has a higher refractive index = positive result and outwards = mineral has a lower refractive index = negative result.

Relief is a more casual way to describe how light is refracted by a given mineral. It is used to describe how well (or poorly) a minerals grains boundaries stand out from its neighbors. If the refractive index is close to the neighboring mineral’s value, then the light will not be bent much either in or out and the boundary between the minerals will be difficult to distinguish. If the refractive indices are very different, then the light will bend quite a bit and the boundary between the minerals will be very distinct. We qualitatively describe relief as “low” or “moderate” or “high” or “very high” and then, if possible, adding whether its “positive” or “negative” relative to the epoxy (positive = higher value, negative = lower value).

If you remember correctly, all anisotropic minerals have two refractive indices depending on how you orient them. As we talked about last week, most of the time the two values aren’t really that far apart from each other and you won’t see the differences in plane light. However, those few cases where double refraction is easy to see in hand sample, we can also see it in plane light as you rotate the stage. For instance, when you watch a calcite grain in plane light as you rotate the stage around, it will vary from a moderate negative relief to a high positive relief.

Minerals can break in a variety of ways depending on how their internal structure is organized. Though, we can’t always see the cleavage or fracture in thin section, when it does appear its as regular semi-parallel lines that are lighter in weight than the mineral boundaries. When we describe a minerals cleavage, we describe the quality (perfect, good, poor), how many planes (lines that are parallel to each other) and at what angle they are to each other. A few examples:

Finally, let’s talk about what color a mineral will be in thin section. If you look at the photomicrographs above in this post (pictures taken with the optical microscope), you’ll notice that there was quite a bit of variation in the color. Color in PPL is due to the absorption of light into the mineral. The light that is not absorbed, is the color we see.

In an isotropic mineral, light is treated the same in all orientations of the mineral, so there will only be one color transmitted. Orientation and how we rotate the stage will still not impact the color.

In anisotropic minerals, how light reacts will depend on the orientation of the mineral. In PPL, we can see this represented as a variation between two colors as you rotate the stage, which is called pleochroism. We can casually describe this as the variation from one color to another (e.g. yellow to brown like the biotite above; blue to purple like the glaucophane below) or we can formally describe the color scheme. To formally define things, we actually have to know more about the orientation of the minerals & be able to use things in crossed polars. Therefore, I’m going to cover the longer pleochroism color scheme in the next segment. See you next week for crossed polars!